Metal‐Surface Interactions and Surface Heterogeneity in ‘Well‐Defined’ Silica‐Supported Alkene Metathesis Catalysts: Evidences and Consequences,Helvetica Chimica Acta

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Metal‐Surface Interactions and Surface Heterogeneity in ‘Well‐Defined’ Silica‐Supported Alkene Metathesis Catalysts: Evidences and Consequences
Helvetica Chimica Acta ( IF 1.5 ) Pub Date : 2020-06-02 , DOI: 10.1002/hlca.202000072
Margherita Pucino ₁ , Wei‐Chih Liao ₁ , Ka Wing Chan ₁ , Erwin Lam ₁ , Roman Schowner ₂ , Pavel A. Zhizhko ₁ , Michael R. Buchmeiser ₂ , Christophe Copéret ₁

Affiliation

Here we present our observations on a peculiar kinetic behavior observed at low catalyst loadings for the metathesis of internal olefins on a highly active silica‐supported tungsten oxo alkylidene prepared through surface organometallic chemistry (SOMC). The observed kinetic profile is comprised of two consecutive regimes, which support the presence of two different active sites on the surface of such catalysts. We propose that the more active site is operational in the beginning of the reaction but deactivates very fast, and the second one initiates over time due to the presence of ethylene formed subsequently under the reaction conditions. A combination of spectroscopic techniques and computations was used to identify the possible nature of these sites. Solid‐state NMR, EXAFS and DFT calculations are consistent with the esence of two distinct surface species that likely originate from the occurrence of slightly different local environments on the surface, that is, metal sites with a different number of nearby siloxane bridges. This data highlights the effects that may arise from the amorphous nature of the supports like silica and the difficulties to obtain truly single‐site catalysts – one of the ultimate goals of SOMC.

中文翻译：

“良好定义的”二氧化硅支撑的烯烃复分解催化剂中的金属表面相互作用和表面异质性：证据和后果

在这里，我们介绍在低催化剂负载下观察到的特殊动力学行为，这些动力学行为是通过表面有机金属化学（SOMC）制备的高活性二氧化硅负载的氧化羰基亚烷基钨上的内烯烃复分解反应的。所观察到的动力学曲线由两个连续的方案组成，这两个方案支持在这种催化剂表面上存在两个不同的活性位点。我们提出，在反应开始时，活性较高的位点是可操作的，但很快失活，第二个位点由于随后在反应条件下形成的乙烯的存在而随时间而引发。光谱技术和计算的结合被用来识别这些位点的可能性质。固态NMR EXAFS和DFT的计算与两种不同表面物种的本质一致，这两种表面物种可能源于表面上局部环境的变化，即附近具有不同数量的硅氧烷桥的金属位点的出现。该数据强调了二氧化硅等载体的无定形性质可能带来的影响，以及难以获得真正的单中心催化剂-这是SOMC的最终目标之一。

更新日期：2020-06-02

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